1. Field of the Invention
The invention relates generally to hydraulic metering devices used to operate downhole devices, such as sliding sleeve valves.
2. Description of the Related Art
A number of downhole devices are operated hydraulically. At times, it is desirable to operate these devices in a stepped manner to respond to changes in downhole conditions. For example, a production tubing string might have a sliding sleeve valve associated with a production nipple to control the flow of fluid into the production tubing string. It would be desirable to be able to shift the sliding sleeve by increments between an open and a closed position. This adjustability would allow fluid flow into the tubing string through the production nipple to be balanced with fluid flowing into the tubing string from other production nipples.
Attempts have been made to use metering devices to adjustably operate a downhole device in a stepped manner. Unfortunately, most of these arrangements have proven to be complex in construction and operation. For example, PCT Application No. PCT/US00/12329 by Schultz et al., entitled “Hydraulic Control System for Downhole Tools” describes a hydraulic control system for multiple well tool assemblies that includes a metering device. The metering device uses two pumps. One of the pumps transfers fluid from a first hydraulic line to an actuator of the well tool assembly in response to fluctuations in pressure on a second hydraulic line, and the other pump transfers fluid from the second hydraulic line to the actuator in response to fluctuations in pressure on the first hydraulic line. The fact that this system requires multiple pumps with associated hydraulic lines makes the system complex in practice and costly.
U.S. Pat. No. 6,585,051 issued to Purkis describes a number of metering apparatuses for use in a downhole environment to discharge a known volume of fluid into a well tool actuator. These metering devices are relatively complex and, therefore, may be prone to failure during use. Additionally, several of the described metering devices incorporate numerous elastomeric O-rings to create fluid tight seals within the metering devices. The O-rings are prone to wear and failure during operation, making metering of a known volume unreliable.
Additionally, the prior art metering arrangements all meter fluid into a fluid input on the downhole device. This can be problematic in some instances
The present invention addresses the problems of the prior art.